Manufacturing methods of the type described above are generally referred to as "fused tapered" methods. In most cases a gas flame is used at the fusing step for fusing together the optical fibers. The gas flame, however, is slow and difficult to adjust. The use of a tungsten glow filament as a source of heat has also been studied, but this, too, requires a vacuum or a shielding gas. Methods for the manufacture of optical couplers are disclosed e.g. in the following publications: FR 850,919, GB 846,274, AU 8,289,666, GB 2,112,165, US 222,113 and US 4,400,055.